Semiconductor device

ABSTRACT

A semiconductor device of stable electrical characteristics, whose oxygen vacancies in a metal oxide is reduced, is provided. The semiconductor device includes a gate electrode, a gate insulating film over the gate electrode, a first metal oxide film over the gate insulating film, a source electrode and a drain electrode which are in contact with the first metal oxide film, and a passivation film over the source electrode and the drain electrode. A first insulating film, a second metal oxide film, and a second insulating film are stacked sequentially in the passivation film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device using an oxidesemiconductor.

In this specification, the semiconductor device refers to any devicethat can operate utilizing semiconductor characteristics. A transistordescribed in this specification is the semiconductor device, and anelectrooptic device, a semiconductor circuit, and an electronic deviceeach including the transistor are all included in the category of thesemiconductor device.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device or a light-emitting display device are formedusing a silicon semiconductor such as amorphous silicon, single crystalsilicon, or polycrystalline silicon provided over a glass substrate.Further, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

Attention has been drawn to a technique in which, instead of the abovesilicon semiconductor, a metal oxide exhibiting semiconductorcharacteristics is used for transistors. In this specification, such ametal oxide exhibiting semiconductor characteristics is also referred toas an oxide semiconductor. For example, a technique is disclosed inwhich a transistor is manufactured using a Zn—O-based oxide or anIn—Ga—Zn—O-based oxide as an oxide semiconductor for application as aswitching element or the like in a pixel of a display device (see PatentDocuments 1 and 2).

Meanwhile, it has been pointed out that hydrogen behaves as a source ofcarriers in an oxide semiconductor. Therefore, some measures need to betaken to prevent hydrogen from entering the oxide semiconductor informing the oxide semiconductor. Further, a shift of a threshold voltagehas been suppressed by reducing the amount of hydrogen contained in notonly the oxide semiconductor but also a gate insulating film in contactwith the oxide semiconductor (see Patent Document 3).

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2007-123861

Patent Document 2: Japanese Published Patent Application No. 2007-096055

Patent Document 3: Japanese Published Patent Application No. 2009-224479

SUMMARY OF THE INVENTION

Further, not only hydrogen, but also an oxygen vacancy functions as asource of carriers in the metal oxide. Some of the oxygen vacancies inthe metal oxide behave as donors to generate electrons that are carriersin the metal oxide. Many oxygen vacancies in a metal oxide including achannel formation region of a transistor lead to generation of electronsin the channel formation region, which shifts the threshold voltage ofthe transistor in the negative direction.

An insulating film which is in contact with the metal oxide filmincluding the channel formation region also affects the thresholdvoltage of the transistor. For example, a negative fixed charge such asan oxygen ion of an unbonded oxygen atom contained in the insulatingfilm enables the threshold voltage of the transistor to be shifted inthe positive direction. In contrast, if oxygen is eliminated from theinsulating film to the outside, the number of negative fixed chargestherein is decreased, so that the threshold voltage of the transistormay be shifted in the negative direction.

In view of the above, one object of one embodiment of the presentinvention is to provide a semiconductor device having excellent andstable electrical characteristics, in which oxygen vacancies in a metaloxide including a channel formation region are reduced and oxygencontained in an insulating film in contact with the metal oxide isprevented from being released to the outside.

To reduce oxygen vacancies in a metal oxide in a transistor, there is atechnique of supplying oxygen into the metal oxide. In one embodiment ofthe present invention, an insulating film from which oxygen iseliminated by heat treatment is provided in contact with a metal oxidefilm including a channel formation region. Accordingly, oxygeneliminated by heat treatment is supplied to the metal oxide to reduceoxygen vacancies therein.

Such an insulating film from which oxygen is eliminated by heattreatment (also referred to as a first insulating film) cannot supplyoxygen sufficiently to the metal oxide in some cases due toout-diffusion of oxygen eliminated at the heat treatment. Against thosecases, in one embodiment of the present invention, in addition to ametal oxide film including a channel formation region (also referred toas a first metal oxide film), another metal oxide film (also referred toas a second metal oxide film) is provided in contact with an insulatingfilm from which oxygen is eliminated by heat treatment. Since the secondmetal oxide film can prevent oxygen from passing therethrough, thesecond metal oxide film can prevent out-diffusion of oxygen eliminatedby the heat treatment. Accordingly, oxygen can be supplied sufficientlyto the first metal oxide film, and further, out-diffusion of oxygencontained in the insulating film can be prevented.

There is also a case where oxygen vacancies exist in the second metaloxide film for preventing oxygen from passing therethrough. Against thatcase, in one embodiment of the present invention, a second metal oxidefilm is sandwiched between insulating films from which oxygen iseliminated by heat treatment (a first insulating film and a secondinsulating film), whereby oxygen vacancies in the second metal oxidefilm can be sufficiently repaired.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode, a gate insulating film over the gateelectrode, a first metal oxide film over the gate insulating film, asource electrode and a drain electrode which are in contact with thefirst metal oxide film, and a passivation film over the source electrodeand the drain electrode. A first insulating film, a second metal oxidefilm, and a second insulating film are stacked sequentially in thepassivation film.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode, a gate insulating film over the gateelectrode, a first metal oxide film over the gate insulating film, asource electrode and a drain electrode which are in contact with thefirst metal oxide film, and a passivation film over the source electrodeand the drain electrode. A second insulating film, a second metal oxidefilm, and a first insulating film are stacked sequentially in the gateinsulating film.

In each of the above-described structures, the thickness of the firstinsulating film may be greater than that of the second insulating film.

One embodiment of the present invention is a semiconductor deviceincluding a base insulating film, a first metal oxide film over the baseinsulating film, a source electrode and a drain electrode which are incontact with the first metal oxide film, a gate insulating film over thefirst metal oxide film, the source electrode, and the drain electrode,and a gate electrode which is provided over the first metal oxide filmwith the gate insulating film interposed therebetween. A firstinsulating film, a second metal oxide film, and a second insulating filmare stacked sequentially in the base insulating film.

One embodiment of the present invention is a semiconductor deviceincluding a base insulating film, a first metal oxide film over the baseinsulating film, a source electrode and a drain electrode which are incontact with the first metal oxide film, a gate insulating film over thefirst metal oxide film, the source electrode, and the drain electrode,and a gate electrode which is provided over the first metal oxide filmwith the gate insulating film interposed therebetween. A secondinsulating film, a second metal oxide film, and a first insulating filmare stacked sequentially in the gate insulating film.

In each of the above-described structures, the thickness of the firstinsulating film is preferably greater than that of the second insulatingfilm.

In each of the above-described structures, the thickness of the firstmetal oxide film may be greater than that of the second metal oxidefilm. As for such a metal oxide film, a thickness as large as 5 nm isrequired to prevent oxygen from passing threrethrough, but too largethickness might lead to an increase of the parasitic capacitance due toits high relative permittivity in the case where the metal oxide film isused except for a metal oxide film including a channel formation region.Therefore, the thickness of the second metal oxide film is preferablygreater than or equal to 5 nm and less than or equal to 15 nm.

In each of the above-described structures, insulating films from whichoxygen is eliminated by heat treatment are preferably used as the firstinsulating film and the second insulating film.

In each of the above-described structures, each of the first metal oxidefilm and the second metal oxide film preferably contains at least twokinds of elements selected from In, Ga, Sn, and Zn. Further, in each ofthe above-described structures, an element contained in the first metaloxide film may be the same as or different from an element contained inthe second metal oxide film. For example, an In—Ga—Zn—O-based materialmay be used for each of the first metal oxide film and the second metaloxide film; alternatively, an In—Ga—Zn—O-based material may be used forthe first metal oxide film, and an In—Ga—Zn—O—N-based material may beused for the second metal oxide film.

Further, the metal oxide film is a conductor, a semiconductor, or aninsulator, which depends on the amount of hydrogen or the number ofoxygen vacancies. For example, the resistivity of the metal oxide filmchanges depending on the amount of hydrogen or the number of oxygenvacancies contained in the metal oxide film.

Heat treatment on the structure in which insulating films from whichoxygen is not eliminated by heat treatment are provided with the metaloxide film interposed therebetween makes the metal oxide film anelectrical conductor. On the other hand, heat treatment on the structurein which insulating films from which oxygen is eliminated by heattreatment are provided with the metal oxide film interposed therebetweenmakes the metal oxide film an electrical insulator. In light of theresistivity of the metal oxide film, a metal oxide film where theresistivity is less than or equal to 10 Ω·cm is a conductor whereas ametal oxide film where the resistivity is greater than or equal to 1×10⁸Ω·cm is an insulator.

In order that the first metal oxide film is a semiconductor, it isnecessary that the resistivity thereof falls within the range over themaximum resistivity of a conductor under the minimum resistivity of aninsulator, and thus the first metal oxide film may be formed to have aresistivity greater than 10 Ω·cm and less than 1×10⁸ Ω·cm.

The first metal oxide film and the second metal oxide film may beamorphous or may have a crystalline state. For example, the first metaloxide film is preferably a non-single-crystal metal oxide including aphase which has triangular, hexagonal, regular triangular, or regularhexagonal atomic arrangement when seen from the direction perpendicularto the a-b plane of the non-single-crystal and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction of the same; in this specification, such a metaloxide film is referred to as a CAAC-OS film (c axis aligned crystallineoxide semiconductor film).

The CAAC-OS film used as the first metal oxide film enables a change ofelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light or application of heat, bias, or thelike to be suppressed, leading to an improvement of the reliability ofthe semiconductor device.

According to one embodiment of the present invention, a semiconductordevice having excellent and stable electrical characteristics can beprovided, in which oxygen vacancies in a metal oxide are reduced andout-diffusion of oxygen contained in an insulating film in contact withthe metal oxide is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views illustrating anexample of a semiconductor device according to one embodiment of thepresent invention;

FIGS. 2A to 2C are views illustrating semiconductor devices according toembodiments of the present invention;

FIGS. 3A to 3E are views illustrating a manufacturing method of asemiconductor device, according to one embodiment of the presentinvention;

FIGS. 4A to 4C are a top view and cross-sectional views illustrating anexample of a semiconductor device according to one embodiment of thepresent invention;

FIGS. 5A to 5C are views illustrating semiconductor devices according toembodiments of the present invention;

FIGS. 6A and 6B are a cross-sectional view and a circuit diagramillustrating an example of a semiconductor device according to oneembodiment of the present invention;

FIG. 7 is a circuit diagram of a semiconductor device according to oneembodiment of the present invention;

FIGS. 8A to 8D are views illustrating a manufacturing method of asemiconductor device, according to one embodiment of the presentinvention;

FIGS. 9A to 9F are views illustrating electronic devices;

FIGS. 10A and 10B show results of withstanding voltage measurement;

FIGS. 11A and 11B show results of withstanding voltage measurement;

FIG. 12 shows results of withstanding voltage measurement;

FIGS. 13A and 13B show results of CV (capacitance vs. voltage)measurement;

FIGS. 14A and 14B show results of CV (capacitance vs. voltage)measurement;

FIG. 15 shows results of CV (capacitance vs. voltage) measurement;

FIG. 16 shows results of TDS; and

FIG. 17 is a view illustrating a structure of Sample according toExample 3.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. However, the present inventionis not limited to the description below, and those skilled in the artwill appreciate that a variety of modifications can be made on the modesand details without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments. Note thatthe same portions or portions having similar functions in the structureof the present invention described below are denoted by the samereference numerals throughout the drawings, and as such descriptionthereof is not repeated.

In each drawing in this specification, the size, the film thickness, orthe region of each component is exaggerated for clarity in some cases.Therefore, embodiments of the present invention are not limited to suchscales.

Further, ordinals such as “first”, “second”, and “third” in thisspecification and the like are used to avoid confusion among components,and do not limit the components numerically. Therefore, for example, theordinal “first” can be replaced with any other ordinal such as “second”or “third” as appropriate.

Still further, respective functions of “source” and “drain” may beswitched upon a change of the direction of a current flow in a circuitoperation, for example. Therefore, the terms “source” and “drain” can beswitched each other in this specification and the like.

Embodiment 1

In this embodiment, a semiconductor device according to one embodimentof the present invention and a manufacturing method thereof aredescribed with reference to FIGS. 1A to 1C, FIGS. 2A to 2C, and FIGS. 3Ato 3E.

<Example of Structure of Semiconductor Device>

FIGS. 1A to 1C are a plan view and cross-sectional views of a transistor200 as an example of a semiconductor device according to one embodimentof the present invention. FIG. 1A is a plan view, FIG. 1B is across-sectional view along A1-A2 of FIG. 1A, and FIG. 1C is across-sectional view along B1-B2 of FIG. 1A. In FIG. 1A, to avoidcomplexity, part of components of the transistor 200 (e.g., a gateinsulating film 104) is omitted.

The transistor 200 shown in FIGS. 1A to 1C includes over a substrate100, a gate electrode 102, a gate insulating film 104 over the gateelectrode 102, a metal oxide film 106 a over the gate insulating film104, and a source and drain electrodes 108 a and 108 b which are incontact with the metal oxide film 106 a. The metal oxide film 106 a isalso referred to as an oxide semiconductor, coming from itssemiconductor characteristics.

The transistor 200 shown in FIGS. 1A to 1C is a bottom-gate transistor,and has a top-contact structure in which the source and drain electrodes108 a and 108 b are in contact with a top surface of the metal oxidefilm 106 a. Alternatively, a bottom-contact structure in which thesource and drain electrodes 108 a and 108 b are in contact with a bottomsurface of the metal oxide film 106 a may be employed.

A region of the metal oxide film 106 a which overlaps with the gateelectrode 102 functions as a channel formation region.

The metal oxide film 106 a is a metal oxide containing at least twoelements selected from In, Ga, Sn, and Zn. The metal oxide may have abandgap greater than or equal to 2 eV and less than 6 eV, preferablygreater than or equal to 2.5 eV and less than or equal to 5.5 eV,further preferably greater than or equal to 3 eV and less than or equalto 5 eV. Such a metal oxide having a wide bandgap enables the off-statecurrent of the transistor 200 to be low.

Further, a passivation film 110 is provided over the metal oxide film106 a and the source and drain electrodes 108 a and 108 b so as to be incontact with the metal oxide film 106 a. In the transistor 200 shown inFIGS. 1A to 1C, the passivation film 110 includes an insulating film112, a metal oxide film 114, and an insulating film 116. In thisembodiment, insulating films from which oxygen is eliminated by heattreatment are used for the insulating films 112 and 116.

In this specification and the like, “oxygen is eliminated by heattreatment” means that the amount of eliminated oxygen (or releasedoxygen) which is converted into oxygen atoms is greater than or equal to1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 3.0×10²⁰atoms/cm³ according to thermal desorption spectroscopy (TDS). Incontrast, “oxygen is not eliminated by heat treatment” means that theamount of eliminated oxygen (or released oxygen) which is converted intooxygen atoms is less than 1.0×10¹⁸ atoms/cm³ according to TDS.

A method for quantifying the amount of released oxygen by conversioninto oxygen atoms with TDS analysis is described below.

The elimination amount of gas in the TDS analysis is proportional to theintegral value of its ion intensity. Thus, the elimination amount of gascan be calculated from the ratio of the measured ion intensity to thereference value of a standard sample. The reference value of a standardsample refers to the ratio of the predetermined density of atomscontained in the sample to the integral value of an ion intensitycorresponding to the atoms.

For example, the number of eliminated oxygen molecules (N_(O2)) from theinsulating film can be obtained according to Formula 1 with the TDSanalysis results of a silicon wafer containing hydrogen at apredetermined density, which is a standard sample, and the TDS analysisresults of the insulating film. Here, all gases having a mass number of32 according to the TDS analysis are assumed to stem from an oxygenmolecule, without consideration of CH₃OH that has the mass number of 32but is less likely to exist in nature. An oxygen molecule containing anoxygen atom having a mass number of 17 or 18 which is an isotope of anoxygen atom is not taken into consideration either because theproportion of such a molecule in natural is minimal.N_(O2)═N_(H2)/S_(H2)×S_(O2)×α  (Formula 1)

In the formula, N_(H2) is the value obtained by converting the number ofhydrogen molecules eliminated from the standard sample into densities,and S_(H2) is the integral value of an ion intensity of the standardsample according to the TDS analysis. Thus, the reference value of thestandard sample is expressed by N_(H2)/S_(H2). Further, S_(O2) is theintegral value of an ion intensity of the insulating film according tothe TDS analysis, and a is a coefficient affecting the ion intensity inthe TDS analysis. For more details of Formula 1, Japanese PublishedPatent Application No. H6-275697 can be referred to. Note that theabove-mentioned value of the amount of eliminated oxygen was obtainedwith a thermal desorption spectroscopy apparatus produced by ESCO Ltd.,EMD-WA1000S/W and with a silicon wafer containing hydrogen atoms at1×10¹⁶ atoms/cm³ as the standard sample.

Further, in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of oxygen molecules. Since theabove-described a includes the ionization rate of oxygen molecules, thenumber of eliminated oxygen atoms can also be estimated through theevaluation of the number of eliminated oxygen molecules.

As described above, N_(O2) is the number of eliminated oxygen molecules.As for the insulating film, the amount of eliminated oxygen which isconverted into oxygen atoms is twice the number of eliminated oxygenmolecules.

An example of a film from which oxygen is eliminated by heat treatmentis a film of silicon oxide containing excess oxygen (SiO_(X) (X>2)). Inthe oxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygenatoms per unit volume is greater than twice the number of silicon atomsper unit volume. The number of silicon atoms and the number of oxygenatoms per unit volume are according to those measured by Rutherfordbackscattering spectrometry.

The metal oxide film 106 a is provided between the gate insulating film104 and the insulating film 112. An insulating film from which oxygen iseliminated by heat treatment is used as the insulating film 112, whereasan insulating film from which oxygen is not eliminated by heat treatmentis used as the gate insulating film 104. By heat treatment, oxygen iseliminated from the insulating film 112 to be supplied to the metaloxide film 106 a.

Further, many negative fixed charges such as an oxygen ion of anunbonded oxygen atom are contained in the insulating film from whichoxygen is eliminated by heat treatment. Thus, the insulating film fromwhich oxygen is eliminated by heat treatment, which is provided incontact with the metal oxide film including a channel formation region,enables the threshold voltage of the transistor to be shifted in thepositive direction, which is preferable.

However, out-diffusion of eliminated oxygen also occurs from theinsulating film at the time of the heat treatment, so that oxygen is notsufficiently supplied to the metal oxide film 106 a in some cases.Further, the out-diffusion of oxygen leads to a reduction in the numberof negative fixed charges in the insulating film. In accordance with thereduction in the number of negative fixed charges, the threshold voltageof the transistor may be shifted in the negative direction.

In view of that case, the metal oxide film 114 which is other than themetal oxide film 106 a is provided on and in contact with the insulatingfilm 112 in one embodiment of the present invention. Since the metaloxide film can prevent oxygen from passing therethrough, out-diffusionof oxygen eliminated from the insulating film 112 at the time of theheat treatment can be prevented.

Further, oxygen vacancies may exist in the metal oxide film 114. In viewof that case, the metal oxide film 114 is sandwiched by the insulatingfilms from which oxygen is eliminated by heat treatment (the insulatingfilms 112 and 116) in one embodiment of the present invention.

The metal oxide film can prevent oxygen from passing therethrough evenwith a thickness as ultrathin as 5 nm. In addition, too large thicknessover 15 nm of the metal oxide film might lead to an increase of theparasitic capacitance due to its high relative permittivity (e.g., 15)in the case where the metal oxide film is used except for a metal oxidefilm including a channel formation region. Therefore, the thickness ofthe metal oxide film 114 is preferably greater than or equal to 5 nm andless than or equal to 15 nm. By suppressing the metal oxide film 114 tobe ultrathin as described above, a remarkable increase of parasiticcapacitance can be prevented even as compared to the case without themetal oxide film 114 in the passivation film.

Since the metal oxide film 114 for preventing out-diffusion of oxygen issandwiched by the insulating films 112 and 116 from which oxygen iseliminated by heat treatment, oxygen is eliminated from the insulatingfilms 112 and 116 by heat treatment to be supplied to the metal oxidefilm 114 to repair oxygen vacancies therein, whereby the metal oxidefilm 114 is turned into an insulator (shows insulating characteristics).Accordingly, the metal oxide film 114, which is used as part of thepassivation film 110, does not affect the electrical characteristics ofthe transistor 200.

To improve the efficiency of supply of oxygen to the metal oxide film106 a, it is preferable that the thickness of the insulating film 112 incontact with the metal oxide film 106 a be greater than that of theinsulating film 116 in contact with the metal oxide film 114. Respectivethicknesses of the insulating films 112 and 116 can be set asappropriate in accordance with the thickness of the passivation film110.

With the film from which oxygen is eliminated by heat treatment providedas the insulating film 112, oxygen is supplied from the insulating film112 to the metal oxide film 106 a, whereby interface states between theinsulating film 112 and the metal oxide film 106 a can be reduced.Accordingly, electric charge or the like that can be generated owing tooperation of the transistor 200 can be prevented from being trapped atthe interface between the insulating film 112 and the metal oxide film106 a, which can make the transistor 200 a transistor with lessdeterioration of electrical characteristics.

Further, with the metal oxide film 114 provided in contact with theinsulating film 112, out-diffusion of oxygen can be prevented, so thatoxygen vacancies in the metal oxide film 106 a including a channelformation region can be sufficiently repaired. Accordingly, a shift inthe negative direction of the threshold voltage of the transistor can besuppressed. In addition, negative fixed charges in the insulating film112 can be prevented from being reduced; accordingly, a shift in thenegative direction of the threshold voltage of the transistor due to areduction in the number of negative fixed charges can be suppressed.

The hydrogen concentration of the metal oxide film 106 a, 114 is lessthan or equal to 1×10²⁰ atoms/cm³, preferably less than or equal to1×10¹⁹ atoms/cm³, further preferably less than or equal to 1×10¹⁸atoms/cm³. Since the hydrogen concentration is low in the channelformation region in the metal oxide film 106 a, a change in thresholdvoltage by light irradiation or BT (bias thermal) stress test issuppressed, leading to a highly reliable transistor whose electricalcharacteristics are stable. As for the metal oxide film 114 which isused not as a semiconductor but as an insulator, it is preferable thatthe hydrogen concentration be as low as possible.

The metal oxide film 114 is a metal oxide containing at least twoelements selected from In, Ga, Sn, and Zn, like the metal oxide film 106a. An element contained in the metal oxide film 114 may be the same asor different from an element contained in the metal oxide film 106 a.For example, an In—Ga—Zn—O-based material may be used for each of themetal oxide film 106 a and the metal oxide film 114; alternatively, anIn—Ga—Zn—O-based material may be used for the metal oxide film 106 a,and an In—Ga—Zn—O—N-based material may be used for the metal oxide film114.

<Application Example of Semiconductor Device>

FIGS. 2A to 2C illustrate cross-sectional structures of transistorshaving different structures from the transistor 200.

A transistor 210 shown in FIG. 2A includes over a substrate 100, a gateelectrode 102, a gate insulating film 120 over the gate electrode 102, ametal oxide film 106 a over the gate insulating film 120, and a sourceand drain electrodes 108 a and 108 b which are in contact with the metaloxide film 106 a.

The transistor 210 is different from the transistor 200 in that a metaloxide film for preventing out-diffusion of oxygen is provided in thegate insulating film 120. That is, the gate insulating film 120 has athree-layer structure including an insulating film 122, a metal oxidefilm 124, and an insulating film 126. Further, an insulating film 118 isprovided as a passivation film over the metal oxide film 106 a and thesource and drain electrodes 108 a and 108 b. In this embodiment,insulating films from which oxygen is eliminated by heat treatment areused for the insulating films 122 and 126, and an insulating film fromwhich oxygen is not eliminated by heat treatment is used as theinsulating film 118.

To improve the efficiency of supply of oxygen to the metal oxide film106 a, it is preferable that the thickness of the insulating film 122 incontact with the metal oxide film 106 a be greater than that of theinsulating film 126 in contact with the metal oxide film 124. Respectivethicknesses of the insulating films 122 and 126 can be set in accordancewith the thickness of the gate insulating film 120. Further, the metaloxide film 124 can prevent oxygen from passing therethrough with athickness equal to or greater than 5 nm, and the thickness of the metaloxide film 124 can be set in accordance with the thickness of the gateinsulating film 120.

A transistor 220 shown in FIG. 2B includes over a substrate 100, a gateelectrode 102, a gate insulating film 120 over the gate electrode 102, ametal oxide film 106 a over the gate insulating film 120, and a sourceand drain electrodes 108 a and 108 b which are in contact with the metaloxide film 106 a. Further, a passivation film 110 is provided over themetal oxide film 106 a and the source and drain electrodes 108 a and 108b.

For the gate insulating film 120 and the passivation film 110 of thetransistor 220, the description of the transistor 200 and the transistor210 can be referred to, and thus detailed description thereof isskipped.

The transistor 200, 210, 220 described hereinabove has a top-contactstructure in which the source and drain electrodes 108 a and 108 b arein contact with a top surface of the metal oxide film 106 a.Alternatively, a bottom-contact structure in which the source and drainelectrodes 108 a and 108 b are in contact with a bottom surface of themetal oxide film 106 a may be employed in a transistor in one embodimentof the present invention. An example of such a bottom-contact structureis illustrated in FIG. 2C.

A transistor 230 shown in FIG. 2C includes over a substrate 100, a gateelectrode 102, a gate insulating film 104 over the gate electrode 102, asource and drain electrodes 108 a and 108 b over the gate insulatingfilm 104, and a metal oxide film 106 a in contact with the source anddrain electrodes 108 a and 108 b. Further, a passivation film 110 isprovided over the metal oxide film 106 a, like the transistor 200.

Since the passivation film 110 is provided to cover a whole of the metaloxide film 106 a, oxygen can be efficiently supplied to the metal oxidefilm 106 a.

Also in the bottom-contact transistor, a metal oxide film for preventingout-diffusion of oxygen may be provided in the gate insulating film orin each of the gate insulating film and the passivation film.

As described above, in one embodiment of the present invention, aninsulating film (first insulating film) from which oxygen is eliminatedby heat treatment is provided in contact with a metal oxide film (firstmetal oxide film) including a channel formation region in order toreduce oxygen vacancies in the first metal oxide film. Further, a metaloxide film (second metal oxide film) which is other than the first metaloxide film is provided in contact with the first metal oxide film. Thesecond metal oxide film is provided between the insulating film (firstinsulating film) from which oxygen is eliminated by heat treatment andan insulating film (second insulating film) from which oxygen iseliminated by heat treatment.

The insulating film 112 (or the insulating film 122) from which oxygenis eliminated by heat treatment is sandwiched by the metal oxide film106 a and the metal oxide film 114 (or the metal oxide film 124),whereby out-diffusion of oxygen eliminated from the insulating film 112(or the insulating film 122) by heat treatment can be prevented, so thatoxygen vacancies in the metal oxide film 106 a can be sufficientlyrepaired. In addition, negative fixed charges contained in theinsulating film 112 (or the insulating film 122) can be prevented fromdecreasing. That is, according to one embodiment of the presentinvention, oxygen vacancies in the metal oxide film 106 a are reduced,and oxygen contained in the insulating film 112 (or the insulating film122) in contact with the metal oxide film 106 a is prevented from beingreleased to the outside, whereby a semiconductor device having excellentand stable electrical characteristics can be provided.

<Manufacturing Method of Semiconductor Device>

Next, as an example of a manufacturing method of a semiconductor deviceof one embodiment of the present invention, a method for manufacturingthe transistor 200 is described with reference to FIGS. 3A to 3E.

First, a conductive film which can be used as a gate electrode is formedover the substrate 100, and then by a photolithography process, a resistmask is formed over the conductive film and the conductive film isetched into an appropriate shape with the use of the resist mask, sothat the gate electrode 102 is formed. Then, the gate insulating film104 is formed over the gate electrode 102 (see FIG. 3A).

Any substrate having an insulating surface can be used as the substrate100. For example, a glass substrate, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, carbon silicon, or the like; a compound semiconductor substrateof silicon germanium or the like; an SOI substrate; or the like can beused as the substrate 100 as long as the substrate has an insulatingsurface, and such a substrate provided with a semiconductor element canbe used as well. Although there is no particular limitation on asubstrate that can be used as the substrate 100, it is necessary thatthe substrate have heat resistance to withstand heat treatment performedlater. In this embodiment, a glass substrate is used as the substrate100.

A flexible substrate can also be used as the substrate 100. In the caseof using a flexible substrate, a transistor may be directly formed overthe flexible substrate, or alternatively, the transistor 200 may beformed over another substrate and then separated from the substrate andtransferred to the flexible substrate. To separate from anothersubstrate and transfer to the flexible substrate, a separation layer andan insulating film may be provided over another substrate and thetransistor 200 may be formed thereover.

Any of single metals such as aluminum, titanium, chromium, nickel,copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten,or an alloy containing any of the metals as its main component can beused as a conductive material for the gate electrode 102. The gateelectrode 102 can be formed of a conductive film having a single-layerstructure or a stacked-layer structure using the above-describedconductive material. For example, a single-layer structure of analuminum film containing silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a two-layer structure inwhich a titanium film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order can be employed. Further, a transparent conductivematerial such as indium oxide or indium oxide including tin oxide, orzinc oxide may be used.

The conductive film which can be used as the gate electrode 102 isformed by a sputtering method, a plasma enhanced CVD method, or the liketo a thickness greater than or equal to 50 nm and less than or equal to300 nm. Then, by a photolithography process, a resist mask is formedover the conductive film and the conductive film is etched into anappropriate shape with the use of the resist mask, so that the gateelectrode 102 is formed. The resist mask can be formed by an ink-jetmethod, a printing method, or the like as appropriate, instead of thephotolithography process. The etching can be performed by a dry etching,a wet etching, or a combination of dry etching and wet etching. In thisembodiment, tungsten is stacked to a thickness of 150 nm by a sputteringmethod as the conductive film.

As the gate insulating film 104, an insulating film(s) selected from anoxide insulating film such as a silicon oxide film, a gallium oxidefilm, or an aluminum oxide film; a nitride insulating film such as asilicon nitride film or an aluminum nitride film; a silicon oxynitridefilm; an aluminum oxynitride film; and a silicon nitride oxide film canbe used. A high-k material such as hafnium oxide, yttrium oxide, hafniumsilicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogenis added (HfSiO_(x)N_(y) (x>0, y>0)), or hafnium aluminate(HfAl_(x)O_(y) (x>0, y>0)) can also be used as well as theabove-described materials. The gate insulating film 104 may have asingle-layer structure of the high-k material or a stacked-layerstructure of the high-k material and the insulating film of theabove-described material.

The gate insulating film 104 is formed by a sputtering method, a plasmaenhanced CVD method, or the like to a thickness greater than or equal to5 nm and less than or equal to 300 nm. With the high-k material, thegate insulating film 104 can be formed thick physically without changingthe electrical thickness (for example, the equivalent silicon oxidethickness) of the gate insulating film, whereby the gate leakage currentcan be reduced.

In this embodiment, a silicon oxynitride film is formed as the gateinsulating film 104 by a plasma enhanced CVD method. A silicon oxidefilm formed by a plasma enhanced CVD method is a film from which oxygenis not eliminated by heat treatment.

Next, a metal oxide film 106 is formed over the gate insulating film 104(see FIG. 3B).

As a material of the metal oxide film 106, a metal oxide materialcontaining two or more selected from In, Ga, Zn, and Sn can be used. Forexample, a four-component metal oxide such as an In—Sn—Ga—Zn—O-basedmaterial; a three-component metal oxide such as an In—Ga—Zn—O-basedmaterial, an In—Sn—Zn—O-based material, an In—Al—Zn—O-based material, aSn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or aSn—Al—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; an In—O-basedmaterial; a Sn—O-based material; a Zn—O-based material; or the like maybe used. Here, for example, the In—Ga—Zn—O-based material means oxidecontaining indium (In), gallium (Ga), and zinc (Zn), and there is noparticular limitation on the composition ratio. Further, theIn—Ga—Zn—O-based oxide material may contain another element in additionto In, Ga, and Zn. It is preferable that the amount of oxygen be inexcess of stoichiometric proportion in the metal oxide film, by whichgeneration of carriers which results from oxygen vacancies in the metaloxide film can be suppressed.

In the case where an In—Ga—Zn—O-based material is used as a material ofthe metal oxide film 106, one example of a target has a compositionratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 (molar ratio). Alternatively, a targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 (molar ratio), atarget having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:4 (molarratio), or a target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=2:1:8(molar ratio) can be used.

In the case where an In—Zn—O-based material is used as a material of themetal oxide film 106, the atomic ratio is set so that In/Zn is greaterthan or equal to 0.5 and less than or equal to 50, preferably greaterthan or equal to 1 and less than or equal to 20, further preferablygreater than or equal to 3/2 and less than or equal to 30/2. The aboverange of the atomic ratio of In to Zn leads to an improvement in thefield-effect mobility of the transistor 200. Here, where the atomicratio of the compound is In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y ispreferably satisfied.

A material represented by InMO₃(ZnO)_(m) (m>0) may be used for the metaloxide film 106. In the formula, M represents one or more metal elementsselected from Ga, Al, Mn, and Co. For example, M may be Ga, Ga and Al,Ga and Mn, Ga and Co, or the like.

The metal oxide film 106 can be formed by a sputtering method, amolecular beam epitaxy method, an atomic layer deposition method, apulsed laser deposition method, or the like. The thickness of the metaloxide film 106 is greater than or equal to 5 nm and less than or equalto 100 nm, preferably greater than or equal to 10 nm and less than orequal to 30 nm. The as-deposited metal oxide film is a semiconductor.

The metal oxide film 106 may be amorphous or may have crystallinity. Forexample, the metal oxide film 106 is a non-single-crystal, specifically,a metal oxide having a phase in which atoms are arranged in a triangle,a hexagon, a regular triangle, or a regular hexagon when seen from thedirection perpendicular to the a-b plane of the non-single-crystal andin which metal atoms or metal atoms and oxygen atoms are arranged inlayers when seen from the direction perpendicular to the c-axis. In thisspecification, such a metal oxide film is referred to as a CAAC-OS film.The use of the CAAC-OS film as the film including a channel formationregion of the transistor 200 enables a change of electricalcharacteristics of the transistor 200 due to irradiation with visiblelight or ultraviolet light, or application of heat, bias, or the like tobe suppressed, improving the reliability of the semiconductor device.

As a method for forming the CAAC-OS film as the metal oxide film 106,for example, there are the following two methods. One is a method inwhich the metal oxide film 106 is deposited while heating the substrate;the other is a method in which film deposition is performed twice andafter each film deposition, heat treatment is performed thereon.

In the method in which the metal oxide film 106 is deposited whileheating the substrate, the substrate temperature is, for example, higherthan or equal to 150° C. and lower than or equal to 450° C., preferablyhigher than or equal to 250° C. and lower than or equal to 350° C. ACAAC-OS film in which the rate of the crystal portion with respect tothe amorphous portion is high can be deposited where the temperature ofthe substrate 100 heated during the deposition of the metal oxide film106 is high.

On the other hand, in the method in which film deposition is performedtwice, a first-layer metal oxide film is formed over the gate insulatingfilm 104 while keeping the temperature of the substrate 100 at atemperature(s) higher than or equal to 100° C. and lower than or equalto 450° C., and then heat treatment is performed thereon at atemperature(s) higher than or equal to 550° C. and lower than the strainpoint of the substrate under an atmosphere of nitrogen, oxygen, a raregas, or dry air. By the heat treatment, a crystalline region (includinga plate-like crystal) is formed in a region including a top surface ofthe first-layer metal oxide film. Then, a second-layer metal oxide filmis formed thicker than the first-layer metal oxide film. After that,heat treatment is performed at a temperature(s) higher than or equal to550° C. and lower than the strain point of the substrate, whereby thecrystal growth proceeds upward using the first-layer metal oxide filmincluding the crystalline region (including the plate-like crystal) inthe region including the top surface as a seed of crystal growth; thus,the entire second-layer metal oxide film is crystallized. The thicknessof the first-layer metal oxide film is preferably greater than or equalto 1 nm and less than or equal to 10 nm.

In the case where the metal oxide film 106 is deposited by a sputteringmethod, it is preferable that hydrogen enter the metal oxide film 106 asless as possible. To prevent hydrogen from entering, a highly purifiedrare gas (typically, argon), highly purified oxygen, or a highlypurified mixed gas of oxygen and a rare gas, from which an impurity suchas a compound or a hydride containing hydrogen, water, or a hydroxylgroup has been removed, is used as appropriate as an atmosphere gassupplied into a process chamber of the sputtering apparatus. Further,for exhaust of the process chamber, a cryopump having high capability ofexhausting water and a sputtering ion pump having high capability ofexhausting hydrogen may be used in combination.

In the above-described manner, the metal oxide film 106 can be depositedwith less entrance of hydrogen. The metal oxide film 106 contains somenitrogen even when the sputtering apparatus is used. For example, thenitrogen concentration of the metal oxide film 106 measured by secondaryion mass spectrometry (SIMS) is less than 5×10¹⁸ atoms/cm³.

Electrical charges are generated in some cases due to oxygen vacanciesin the metal oxide film 106 during or after the deposition of the metaloxide film 106. In general, part of oxygen vacancies in a metal oxidefilm becomes a donor to generate an electron as a carrier. That is, alsoin the transistor 200, part of oxygen vacancies in the metal oxide film106 becomes a donor to generate an electron as a carrier, which shiftsthe threshold voltage of the transistor 200 in the negative direction.In addition, the generation of an electron in the metal oxide film 106is more likely to occur in oxygen vacancies in the vicinity of theinterface between the metal oxide film 106 and the gate insulating film104.

Hence, a first heat treatment is performed thereon after the depositionof the metal oxide film 106.

The first heat treatment is performed to discharge hydrogen (compoundincluding water or a hydroxyl group) from the metal oxide film. That is,hydrogen, which is an unstable carrier source, is eliminated from themetal oxide film 106 by the first heat treatment, whereby a shift of thethreshold voltage of the transistor 200 in the negative direction issuppressed. Further, the reliability of the transistor 200 can beimproved.

The first heat treatment is performed at a temperature(s) higher than orequal to 150° C. and lower than the strain point of the substrate,preferably higher than or equal to 250° C. and lower than or equal to450° C., further preferably higher than or equal to 300° C. and lowerthan or equal to 450° C., in an oxidation atmosphere or an inertatmosphere. The oxidation atmosphere refers to an atmosphere includingan oxidation gas such as oxygen, ozone, or nitrogen oxide at aconcentration of 10 ppm or more. The inert atmosphere refers to anatmosphere including the oxidation gas at a concentration of less than10 ppm and is filled with nitrogen or a rare gas. The process time ofthe heat treatment is 3 minutes to 24 hours. Heat treatment for a timelonger than 24 hours is not preferable because the productivity isreduced.

There is no particular limitation on a heat treatment apparatus used forthe first heat treatment, and the apparatus may be equipped with adevice for heating an object to be processed by heat conduction or heatradiation from a heating element such as a resistance heating element.For example, an electric furnace, or a rapid thermal annealing (RTA)apparatus such as a lamp rapid thermal annealing (LRTA) apparatus or agas rapid thermal annealing (GRTA) apparatus can be used. The LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. TheGRTA apparatus is an apparatus for performing heat treatment using ahigh-temperature gas.

Next, a resist mask is formed over the metal oxide film 106 by aphotolithography process, and the metal oxide film 106 is etched withthe resist mask into a desired shape, so that the island-shaped metaloxide film 106 a is formed (see FIG. 3C). The resist mask can be formedby an ink-jet method, a printing method, or the like as appropriate, aswell as the photolithography process. It is preferable to etch the metaloxide film 106 so that an end portion of the metal oxide film 106 a istapered, by which the coverage with any film formed in the followingmanufacturing process of the transistor 200 can be improved to preventthe film from being cut by a step. The end portion of the metal oxidefilm 106 a can be tapered by etching while the resist mask is made torecede.

The etching step may be performed by dry etching, wet etching, orcombination thereof. As an etchant used for the wet etching, a mixedsolution of phosphoric acid, acetic acid, and nitric acid, an ammoniahydrogen peroxide mixture (hydrogen peroxide water at 31 wt %:ammoniawater at 28 wt %:water=5:2:2 (volume ratio)), or the like can be used.Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may beused.

As an etching gas for the dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used. Alternatively, a gas containing fluorine(fluorine-based gas such as carbon tetrafluoride (CF₄), sulfurhexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane(CHF₃)); hydrogen bromide (HBr); oxygen (O₂); any of these gases towhich a rare gas such as helium (He) or argon (Ar) is added; or the likecan be used.

As the dry etching, a parallel-plate reactive ion etching (RIE) method,an inductively coupled plasma (ICP) etching method, or the like can beused. In order to process the film into a desired shape, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

Next, a conductive film which can be used for a source electrode and adrain electrode is deposited over the metal oxide film 106 a, a resistmask is formed over the conductive film by a photolithography process,and the conductive film is etched with the resist mask into a desiredshape, so that the source and drain electrodes 108 a and 108 b areformed (see FIG. 3D). The similar conductive material as the conductivematerial for the gate electrode 102 can be used for the source and drainelectrodes 108 a and 108 b.

In this embodiment, the source and drain electrodes 108 a and 108 b areformed as follows: a 50-nm-thick titanium film, a 100-nm-thick aluminumfilm, and a 50-nm-thick titanium film are formed by a sputtering method,and then subjected to a photolithography process and an etching process.

Next, the passivation film 110 is formed over the metal oxide film 106 aand the source and drain electrodes 108 a and 108 b (see FIG. 3E). Inthis embodiment, the insulating film 112, the metal oxide film 114, andthe insulating film 116 are formed in this order to form the passivationfilm 110.

Insulating films selected from a silicon oxide film, a gallium oxidefilm, an aluminum oxide film, a silicon oxynitride film, an aluminumoxynitride film, and the like can be used as the insulating films 112and 116. Any of the insulating films 112 and 116 can be formed by amethod similar to that of the gate insulating film 104.

The metal oxide film 114 can be formed using a material and a methodwhich are similar to those of the metal oxide film 106, and as suchdetailed description thereof is skipped.

The thickness of the passivation film 110 may be greater than or equalto 50 nm and less than or equal to 1000 nm, preferably greater than orequal to 100 nm and less than or equal to 300 nm.

In this embodiment, a 200-nm-thick silicon oxide film is deposited asthe insulating film 112 by a sputtering method, a 5-nm-thickIn—Ga—Zn—O-based metal oxide film is deposited as the metal oxide film114 by a sputtering method, and a 50-nm-thick silicon oxide film isdeposited as the insulating film 116 by a sputtering method.

In the case where the insulating film 112 and the insulating film 116are deposited by a sputtering method, it is preferable that hydrogenenters the insulating film 112 and the insulating film 116 as less aspossible. To prevent hydrogen from entering, a highly purified rare gas(typically, argon), highly purified oxygen, or a highly purified mixedgas of oxygen and a rare gas, from which an impurity such as a compoundcontaining hydrogen, water, or a hydroxyl group has been removed, isused as appropriate as an atmosphere gas supplied into a process chamberof the sputtering apparatus. Further, for exhaust of the processchamber, a cryopump having high capability of exhausting water and asputtering ion pump having high capability of exhausting hydrogen may beused in combination.

With the first heat treatment, oxygen might be eliminated from a topsurface of the metal oxide film 106 a, in addition to a release ofhydrogen from the metal oxide film 106 a. Accordingly, oxygen vacanciesmay be generated in the metal oxide film 106 a. To repair such generatedoxygen vacancies, a second heat treatment is preferably performed afterthe deposition of the passivation film 110.

The condition and apparatus of the first heat treatment may be appliedto the second heat treatment appropriately, and as such detaileddescription thereof is skipped.

With the second heat treatment, oxygen is eliminated from the insulatingfilm 112 to be supplied to the metal oxide film 106 a. Since the metaloxide film 114 for preventing out-diffusion of oxygen is provided overthe insulating film 112, out-diffusion of oxygen contained in theinsulating film 112 at the time of the second heat treatment can beprevented, whereby oxygen can be supplied effectively to the metal oxidefilm 106 a. The insulating films 112 and 116 supply oxygen also to themetal oxide film 114, so that oxygen vacancies in the metal oxide film114 can be repaired, whereby the resistance of the metal oxide film 114is increased; thus, the metal oxide film 114 is turned into an insulator(shows insulating characteristics). Accordingly, the metal oxide film114, which is used in the passivation film 110 in this embodiment, doesnot affect electrical characteristics of the transistor 200.

With the first heat treatment and the second heat treatment, the metaloxide films 106 a and 114 are highly purified by reducing the hydrogenconcentration therein. The hydrogen concentration of any of the metaloxide film 106 a and the metal oxide film 114 is less than or equal to1×10²⁰ atoms/cm³, preferably less than or equal to 1×10¹⁹ atoms/cm³,further preferably less than or equal to 1×10¹⁸ atoms/cm³. As for themetal oxide film 114 which is used not as a semiconductor but as aninsulator, it is preferable that the hydrogen concentration be as low aspossible. The hydrogen concentrations of the metal oxide film 106 a andthe metal oxide film 114 are measured by SIMS.

The metal oxide film 106 a which is highly purified by sufficientlyreducing the hydrogen concentration and where defect states in theenergy gap generated due to oxygen vacancies are reduced by supplying asufficient amount of oxygen by the first heat treatment and the secondheat treatment enables the off-state current of the transistor 200 to bereduced. Specifically, the off-state current (per unit channel width (1μm) in this embodiment) at room temperature (25° C.) is suppressed to100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA orless.

Further, alkali metal such as lithium (Li) or sodium (Na) is an impurityfor the metal oxide film 106 a and the metal oxide film 114, and thuspreferably contained as less as possible. The concentration of alkalimetal in any of the metal oxide film 106 a and the metal oxide film 114is preferably 2×10¹⁶ atoms/cm³ or less, further preferably 1×10¹⁵atoms/cm³ or less. Likewise, alkaline earth metal is also an impurityand thus preferably contained as less as possible.

Further, the metal oxide film is a conductor, a semiconductor, or aninsulator, which depends on the amount of hydrogen or the number ofoxygen vacancies. For example, the resistivity of the metal oxide filmchanges depending on the amount of hydrogen or the number of oxygenvacancies contained in the metal oxide film.

Heat treatment (for example, at 350° C.) on the structure in whichinsulating films from which oxygen is not eliminated by heat treatmentare provided with the metal oxide film interposed therebetween decreasesthe resistivity of the metal oxide film to 10 Ω·cm or less, whereby themetal oxide film is turned into a conductor. On the other hand, heattreatment (for example, at 350° C.) on the structure in which insulatingfilms from which oxygen is eliminated by heat treatment are providedwith the metal oxide film interposed therebetween increases theresistivity of the metal oxide film to 1×10⁸ Ω·cm or more, whereby themetal oxide film is turned into an insulator. Therefore, in order thatthe metal oxide film 114 is an insulator, the metal oxide film 114 maybe formed to have a resistivity greater than or equal to 1×10⁸ Ω·cm.

In order that the metal oxide film 106 a is a semiconductor, it isnecessary that the resistivity thereof falls within the range over themaximum resistivity of a conductor under the minimum resistivity of aninsulator, and thus the metal oxide film 106 a may be formed to have aresistivity greater than 10 Ω·cm and less than 1×10⁸ Ω·cm.

Through the above-described process, the transistor 200 can bemanufactured (see FIG. 3E).

The insulating film from which oxygen is eliminated by heat treatment isprovided in contact with the metal oxide film (oxide semiconductor)including a channel formation region, and the metal oxide film forpreventing out-diffusion of oxygen is provided in contact with theinsulating film, whereby out-diffusion of oxygen of the insulating filmcan be suppressed and oxygen can be supplied efficiently to the metaloxide film including the channel formation region. Accordingly, oxygenvacancies in the metal oxide film including the channel formation regioncan be reduced, whereby generation of electrons which are carriers canbe suppressed, so that a shift of the threshold voltage of thetransistor in the negative direction can be suppressed.

Further, the metal oxide film for preventing out-diffusion of oxygen issandwiched by the insulating films from which oxygen is eliminated byheat treatment, and the heat treatment is performed thereon, wherebyoxygen vacancies in the metal oxide film for preventing out-diffusion ofoxygen can be reduced, so that the metal oxide film can be turned intoan insulator (shows insulating characteristics).

<Manufacturing Method of Application Example of Semiconductor Device>

The transistor 210 shown in FIG. 2A can be manufactured in a mannerdescribed below.

The gate electrode 102 is formed over the substrate 100, and then, thegate insulating film 120 is formed thereon. The insulating film 126, themetal oxide film 124, and the insulating film 122 are stacked in thisorder in the gate insulating film 120.

Materials and formation methods of the insulating films 126 and 122 aresimilar to those of the insulating films 112 and 116. Further, amaterial and a formation method of the metal oxide film 124 are similarto those of the metal oxide film 114.

Next, a first heat treatment is preferably performed thereon.Accordingly, the metal oxide film 124, which is sandwiched by theinsulating films 126 and 122 from which oxygen is eliminated by heattreatment, is turned into an insulator (shows insulatingcharacteristics). Then, a metal oxide film is formed over the gateinsulating film 120 and is subjected to a photolithography process andan etching process to form the metal oxide film 106 a.

Next, a conductive film is formed over the metal oxide film 106 a and issubjected to a photolithography process and an etching process to formthe source and drain electrodes 108 a and 108 b.

Next, the insulating film 118 is formed over the metal oxide film 106 aand the source and drain electrodes 108 a and 108 b. A material and aformation method of the insulating film 118 are similar to those of theinsulating film 112. Then, a second heat treatment may be performedthereon.

Through the above-described process, the transistor 210 can bemanufactured.

The transistor 220 shown in FIG. 2B can be manufactured in a mannerdescribed below.

The gate electrode 102 is formed over the substrate 100, and then, thegate insulating film 120 is formed thereon.

Next, a metal oxide film is formed over the gate insulating film 120 andis subjected to a photolithography process and an etching process toform the metal oxide film 106 a. After that, a first heat treatment isperformed thereon. Accordingly, oxygen eliminated from the insulatingfilm 126 is supplied to the metal oxide film 124, and oxygen eliminatedfrom the insulating film 122 is supplied to the metal oxide films 124and 106 a. Further, hydrogen, water, and the like in the metal oxidefilm 106 a can be reduced.

Next, the source and drain electrodes 108 a and 108 b and thepassivation film 110 are formed over the metal oxide film 106 a. Then, asecond heat treatment is performed thereon.

Through the above, the transistor 220 can be manufactured.

Embodiment 1 can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 2

In this embodiment, a transistor having a structure different from thestructure described in Embodiment 1 is described.

FIGS. 4A to 4C are a plan view and cross-sectional views of a transistor400 as an example of a semiconductor device according to one embodimentof the present invention. FIG. 4A is a plan view, FIG. 4B is across-sectional view along A1-A2 of FIG. 4A, and FIG. 4C is across-sectional view along B1-B2 of FIG. 4A. In FIG. 4A, to avoidcomplexity, part of components of the transistor 400 (e.g., a gateinsulating film 304) is omitted.

The transistor 400 shown in FIGS. 4A to 4C includes over a substrate300, a base insulating film 310, a metal oxide film 306 a over the baseinsulating film 310, a source and drain electrodes 308 a and 308 b whichare in contact with the metal oxide film 306 a, a gate insulating film304 over the metal oxide film 306 a and the source and drain electrodes308 a and 308 b, and a gate electrode 302 which is provided over thegate insulating film 304 so as to overlap with the metal oxide film 306a.

A substrate similar to the substrate 100 can be used as the substrate300. The metal oxide film 306 a can be formed using a material and amethod which are similar to those of the metal oxide film 106 a. Thesource and drain electrodes 308 a and 308 b can be formed using amaterial and a method which are similar to those of the source and drainelectrodes 108 a and 108 b. The gate insulating film 304 can be formedusing a material and a method which are similar to those of the gateinsulating film 104. The gate electrode 302 can be formed using amaterial and a method which are similar to those of the gate electrode102.

The transistor 400 shown in FIGS. 4A to 4C is a top-gate transistor, andhas a top-contact structure in which the source and drain electrodes 308a and 308 b are in contact with a top surface of the metal oxide film306 a. Alternatively, a bottom-contact structure in which the source anddrain electrodes 308 a and 308 b are in contact with a bottom surface ofthe metal oxide film 306 a may be employed.

A region of the metal oxide film 306 a which overlaps with the gateelectrode 302 functions as a channel formation region.

The base insulating film 310 is provided over the substrate 300 so as tobe in contact with the metal oxide film 306 a. In the transistor 400shown in FIGS. 4A to 4C, the base insulating film 310 includes aninsulating film 312, a metal oxide film 314, and an insulating film 316.In this embodiment, insulating films from which oxygen is eliminated byheat treatment are used for the insulating films 312 and 316, and aninsulating film from which oxygen is not eliminated by heat treatment isused for the gate insulating film 304.

The metal oxide film 306 a is sandwiched by the gate insulating film 304and the insulating film 312. Since the insulating film from which oxygenis eliminated by heat treatment is used as the insulating film 312,oxygen is eliminated from the insulating film 312 to be supplied to themetal oxide film 306 a by heat treatment.

Further, in one embodiment of the present invention, the metal oxidefilm 314 for preventing out-diffusion of oxygen is provided in contactwith the insulating film 312 from which oxygen is eliminated by heattreatment. Accordingly, out-diffusion of oxygen eliminated from theinsulating film 312 by the heat treatment can be prevented.

Further, in one embodiment of the present invention, the metal oxidefilm 314 for preventing out-diffusion of oxygen is sandwiched by theinsulating films 312 and 316 from which oxygen is eliminated by heattreatment. Accordingly, oxygen is eliminated also from the insulatingfilm 316 by heat treatment to be supplied to the metal oxide film 314.Oxygen vacancies in the metal oxide film 314 are repaired by oxygensupplied from the insulating films 312 and 316, whereby the metal oxidefilm 314 is turned into an insulator (shows insulating characteristics).Accordingly, the metal oxide film 314, which is used as part of the baseinsulating film 310, does not affect the electrical characteristics ofthe transistor 400.

To improve the efficiency of supply of oxygen to the metal oxide film306 a, it is preferable that the thickness of the insulating film 312 incontact with the metal oxide film 306 a be greater than that of theinsulating film 316 in contact with the metal oxide film 314. Respectivethicknesses of the insulating films 312 and 316 can be set in accordancewith the thickness of the base insulating film 310.

The insulating film 312 can be formed using a material and a methodwhich are similar to those of the insulating film 112. The metal oxidefilm 314 can be formed using a material and a method which are similarto those of the metal oxide film 114. Further, the insulating film 316can be formed using a material and a method which are similar to thoseof the insulating film 116.

With the film from which oxygen is eliminated by heat treatment providedas the insulating film 312, oxygen is supplied from the insulating film312 to the metal oxide film 306 a, whereby interface states between theinsulating film 312 and the metal oxide film 306 a can be reduced.Accordingly, trapping of electric charges or the like, that can begenerated owing to operation of the transistor 400, at the interfacebetween the insulating film 312 and the metal oxide film 306 a can besuppressed, which can make the transistor 400 a transistor with lessdeterioration of electrical characteristics.

The metal oxide film 314 is a metal oxide containing at least twoelements selected from In, Ga, Sn, and Zn, like the metal oxide film 306a. An element contained in the metal oxide film 314 may be the same asor different from an element contained in the metal oxide film 306 a.For example, an In—Ga—Zn—O-based material may be used for each of themetal oxide film 306 a and the metal oxide film 314; alternatively, anIn—Ga—Zn—O-based material may be used for the metal oxide film 306 a,and an In—Ga—Zn—O—N-based material may be used for the metal oxide film314.

<Application Example of Semiconductor Device>

FIGS. 5A to 5C illustrate cross-sectional structures of transistorshaving different structures from the transistor 400.

A transistor 410 shown in FIG. 5A includes over a substrate 300, aninsulating film 318 as a base insulating film, a metal oxide film 306 aover the insulating film 318, a source and drain electrodes 308 a and308 b which are in contact with the metal oxide film 306 a, a gateinsulating film 320 over the metal oxide film 306 a and the source anddrain electrodes 308 a and 308 b, and a gate electrode 302 which isprovided over the gate insulating film 320 so as to overlap with achannel formation region of the metal oxide film 306 a.

The transistor 410 is different from the transistor 400 in that a metaloxide film for preventing out-diffusion of oxygen is provided in thegate insulating film 320. That is, the gate insulating film 320 has athree-layer structure including an insulating film 322, a metal oxidefilm 324, and an insulating film 326. Further, the insulating film 318is provided as the base insulating film. In this embodiment, insulatingfilms from which oxygen is eliminated by heat treatment are used for theinsulating films 322, 326, and 318.

To improve the efficiency of supply of oxygen to the metal oxide film306 a, it is preferable that the thickness of the insulating film 322 incontact with the metal oxide film 306 a be greater than that of theinsulating film 326 in contact with the metal oxide film 324. Respectivethicknesses of the insulating films 322 and 326 can be set in accordancewith the thickness of the gate insulating film 320. Further, the metaloxide film 324 can prevent oxygen from passing therethrough with athickness equal to or greater than 5 nm, and the thickness of the metaloxide film 324 can be set in accordance with the thickness of the gateinsulating film 320.

A transistor 420 shown in FIG. 5B includes over a substrate 300, a baseinsulating film 310, a metal oxide film 306 a over the base insulatingfilm 310, a source and drain electrodes 308 a and 308 b which are incontact with the metal oxide film 306 a, a gate insulating film 320 overthe metal oxide film 306 a and the source and drain electrodes 308 a and308 b, and a gate electrode 302 which is provided over the gateinsulating film 320 so as to overlap with a channel formation region ofthe metal oxide film 306 a.

For the base insulating film 310 and the gate insulating film 320 of thetransistor 420, the description of the transistor 400 and the transistor410 can be referred to, and thus detailed description thereof isskipped.

The transistor 400, 410, 420 described hereinabove has a top-contactstructure in which the source and drain electrodes 308 a and 308 b arein contact with a top surface of the metal oxide film 306 a.Alternatively, a bottom-contact structure in which the source and drainelectrodes 308 a and 308 b are in contact with a bottom surface of themetal oxide film 306 a may be employed in a transistor in one embodimentof the present invention. An example of such a bottom-contact structureis illustrated in FIG. 5C.

A transistor 430 shown in FIG. 5C includes over a substrate 300, a baseinsulating film 310, a source and drain electrodes 308 a and 308 b overthe base insulating film 310, a metal oxide film 306 a which is providedin contact with the source and drain electrodes 308 a and 308 b, a gateinsulating film 304 over the source and drain electrodes 308 a and 308 band the metal oxide film 306 a, and a gate electrode 302 which isprovided so as to overlap with a channel formation region of the metaloxide film 306 a.

As described above, a transistor according to one embodiment of thepresent invention can have various embodiments.

Furthermore, the structures, methods, and the like described in thisembodiment can be combined as appropriate with any of the structures,methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, examples of a semiconductor device using thetransistor described in the above embodiment and a manufacturing methodthereof, and examples of a circuit configuration of the semiconductordevice and operation thereof are described with reference to FIGS. 6Aand 6B, FIG. 7, and FIGS. 8A to 8D. In this embodiment, an example of asemiconductor device whose structure corresponds to that of a so-calleddynamic random access memory (DRAM) is described. In each circuitdiagram, in some cases, a transistor using an oxide semiconductor ismarked with “OS”.

<Cross-Sectional Structure of Semiconductor Device>

First, an example of a cross-sectional structure of a semiconductordevice is described with reference to FIG. 6A. The semiconductor deviceshown in FIG. 6A includes a transistor 400 and a capacitor 402.

A transistor according to one embodiment of the present invention isused as the transistor 400 in FIG. 6A. The transistor 400 includes overa substrate 300, a base insulating film 310, a metal oxide film 306 aover the base insulating film 310, a source and drain electrodes 308 aand 308 b, a gate insulating film 304, and a gate electrode 302 a. Thebase insulating film 310 includes an insulating film 312, a metal oxidefilm 314, and an insulating film 316.

The capacitor 402 in FIG. 6A includes the gate insulating film 304, thesource or drain electrode 308 a, and an electrode 302 b. The source ordrain electrode 308 a functions as one electrode of the capacitor 402,and the electrode 302 b functions as the other electrode of thecapacitor 402.

An insulating film 330 is provided to cover the transistor 400 and thecapacitor 402. Further, a wiring 332 is connected to the source or drainelectrode 308 b via an opening formed in the insulating film 330.

<Basic Circuit 1>

Next, a basic circuit configuration of the semiconductor device shown inFIG. 6A and its operation are described with reference to FIG. 6B. In asemiconductor device shown in FIG. 6B, a first wiring (1st Line) iselectrically connected to one of the source and drain electrodes of thetransistor 400, a second wiring (2nd Line) is electrically connected toa gate electrode of the transistor 400, and one electrode of thecapacitor 402 is electrically connected to the other of the source anddrain electrodes of the transistor 400. Further, a third wiring (3rdLine) is electrically connected to the other electrode of the capacitor402.

A transistor using an oxide semiconductor is used as the transistor 400,for example. Such a transistor using an oxide semiconductor has afeature of a significantly small off-state current. Therefore, with thetransistor 400 turned off, a potential supplied to the capacitor 402 canbe retained for an extremely long time.

The semiconductor device shown in FIG. 6B utilizes its feature in thatthe potential supplied to the capacitor 402 can be retained, wherebywriting, retaining, and reading of data can be performed in a mannerdescribed below.

Writing and retaining of data are described below. For simplicity, thepotential of the third line is assumed to be fixed. First, the potentialof the second wiring is set to a potential which allows the transistor400 to be turned on to turn on the transistor 400. Consequently, thepotential of the first wiring is supplied to one electrode of thecapacitor 402. That is, predetermined electric charge is supplied to thecapacitor 402 (data writing). After that, the potential of the secondline is changed to a potential which allows the transistor 400 to beturned off to turn off the transistor 400, whereby the electric chargegiven to the capacitor 402 is retained (data retaining). The transistor400 whose off-state current is extremely small as described aboveenables electric charge to be retained for a long time.

Next, reading of data is described. The potential of the second line ischanged to the potential which allows the transistor 400 to be turned onwhile the predetermined potential (constant potential) is supplied tothe first wiring, so that the potential of the first wiring variesdepending on the amount of electric charge retained in the capacitor402. Therefore, the retained data can be read by detecting the potentialof the first wiring.

Next, rewriting of data is described. Data rewriting is performed in amanner similar to that of the writing and retaining of data. That is,the potential of the second wiring is set to the potential which allowsthe transistor 400 to be turned on to turn on the transistor 400.Accordingly, the potential of the first wiring (potential related to newdata) is supplied to one electrode of the capacitor 402. After that, thepotential of the second wiring is changed to the potential which allowsthe transistor 400 to be turned off to turn off the transistor 400, sothat the electric charge related to new data is retained in thecapacitor 402.

As described above, in the semiconductor device according to oneembodiment of the present invention, data can be directly rewritten bywriting the data. Accordingly, high-speed operation of the semiconductordevice can be realized.

An n-channel transistor (n-type transistor) in which electrons arecarriers is used in the above description, but it will be appreciatedthat a p-channel transistor in which holes are majority carriers canalternatively be used instead of the n-channel transistor.

FIG. 7 illustrates an example of a circuit diagram of a semiconductordevice including m×n memory cells 450. The configuration of the memorycell 450 in FIG. 7 is similar to that in FIGS. 6A and 6B. In otherwords, the first wiring in FIG. 6B corresponds to a bit line BL in FIG.7; the second wiring in FIG. 6B corresponds to a word line WL in FIG. 7;and the third wiring in FIG. 6B corresponds to a source line SL in FIG.7 (see FIG. 7).

The semiconductor device shown in FIG. 7 includes n bit lines BL, m wordlines WL, a memory cell array having the memory cells 450 arranged in amatrix of m (rows) (in the vertical direction)×n (columns) (in thehorizontal direction), a first driver circuit 461 connected to the n bitlines BL, and a second driver circuit 462 connected to the m word linesWL.

The memory cell 450 includes a transistor 400 and a capacitor 402. Agate electrode of the transistor 400 is connected to the word line WL.Further, one of a source and drain electrodes of the transistor 400 isconnected to the bit line BL, and the other of the source and drainelectrodes of the transistor 400 is connected to one electrode of thecapacitor 402. The other electrode of the capacitor 402 is connected tothe source line SL and supplied with a predetermined potential. Thetransistor described in the above embodiment is used as the transistor400.

The semiconductor device which is one embodiment of the presentinvention is a transistor in which a channel formation region is formedin an oxide semiconductor, and thus has a feature in that the off-statecurrent is smaller than that of a transistor in which a channelformation region is formed in single crystal silicon. Accordingly, withthe transistor applied to the semiconductor device shown in FIG. 7,which is regarded as a so-called DRAM, a memory whose interval betweenrefresh periods is extremely long can be provided.

<Manufacturing Method of Semiconductor Device>

Next, a method for manufacturing the semiconductor device shown in FIGS.6A and 6B is described with reference to FIGS. 8A to 8D.

First, over the substrate 300, the insulating film 312, the metal oxidefilm 314, and the insulating film 316 are stacked in this order to formthe base insulating film 310 (see FIG. 8A). A material similar to thesubstrate 100 can be used for the substrate 300, and thus detaileddescription thereof is skipped. For the insulating films 312 and 316,respectively, the description of the insulating films 112 and 116 can bereferred to, and thus detailed description thereof is skipped.

Next, the metal oxide film 306 a is formed over the base insulating film310 (see FIG. 8B). For the metal oxide film 306 a, the description ofthe metal oxide film 106 a can be referred to.

Next, the source and drain electrodes 308 a and 308 b are formed incontact with the metal oxide film 306 a, and the gate insulating film304 is formed over the source and drain electrodes 308 a and 308 b.Then, over the gate insulating film 304, the gate electrode 302 a isformed in a region which overlaps with a channel formation region in themetal oxide film 306 a, and the electrode 302 b is formed in a regionwhich overlaps with the source or drain electrode 308 a (see FIG. 8C).For the source and drain electrodes 308 a and 308 b, the description ofthe source and drain electrodes 108 a and 108 b can be referred to.

Next, the insulating film 330 functioning as an interlayer insulatingfilm is formed to cover the gate insulating film 304, the gate electrode302 a, and the electrode 302 b. Then, an opening is formed in theinsulating film 330 and the gate insulating film 304, and the wiring 332is formed over the insulating film 330, whereby the source or drainelectrode 308 b is electrically connected to the wiring 332.

As the insulating film 330 functioning as an interlayer insulating film,an inorganic material (e.g., silicon oxide, silicon nitride, or siliconoxynitride), a photosensitive or non-photosensitive organic material(polyimide, acrylic, polyamide, polyimide amide, resist, orbenzocyclobutene), a material called siloxane, which is composed of askeleton formed by the bond of silicon (Si) and oxygen (O) and containsat least hydrogen or at least one of fluorine, an alkyl group, andaromatic hydrocarbon as a substituent, or a stack thereof can be used.

The wiring 332 is formed as follows: a conductive film is formed by asputtering method, a plasma enhanced CVD method, or the like and issubjected to a photolithography process and an etching process. As amaterial of the conductive film, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten, an alloycontaining any of these elements as a component, or the like can beused. One or more materials selected from manganese, magnesium,zirconium, beryllium, neodymium, and scandium may be used as well. Thedetails thereof are similar to those of the gate electrode 102 and thelike.

Through the above process, the semiconductor device including thetransistor 400 and the capacitor 402 can be manufactured (see FIG. 8D).

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

A semiconductor device according to one embodiment of the presentinvention can be applied to a variety of electronic devices (includinggame machines). Examples of the electronic devices are a television set(also called a television or a television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone handset (also calleda mobile phone or a mobile phone device), a portable game machine, aportable data assistance, an audio reproducing device, a large-sizedgame machine such as a pachinko machine, and the like. Examples of suchelectronic appliances having the semiconductor device described in theabove embodiment are described below.

FIG. 9A is a laptop personal computer which includes a main body 3001, ahousing 3002, a display portion 3003, a keyboard 3004, and the like. Anysemiconductor device described in Embodiments 1 and 2 can be applied tothe display portion 3003. Further, the semiconductor device described inEmbodiment 3 can be applied to a memory circuit in the housing 3002.Since a change of the electrical characteristics of any semiconductordevice described in Embodiments 1 to 3 is suppressed, a laptop personalcomputer with high reliability can be provided.

FIG. 9B is a portable data assistance (PDA) which includes a displayportion 3023, an external interface 3025, an operation button 3024, andthe like provided for a main body 3021. A stylus 3022 is equipped as anaccessory for operation. Any semiconductor device described inEmbodiments 1 and 2 can be applied to the display portion 3023. Further,the semiconductor device described in Embodiment 3 can be applied to amemory circuit in the main body 3021. Since a change of the electricalcharacteristics of any semiconductor device described in Embodiments 1to 3 is suppressed, a portable data assistance with high reliability canbe provided.

FIG. 9C illustrates an example of an e-book reader. For example, ane-book reader includes two housings, a housing 2701 and a housing 2703.The housing 2701 and the housing 2703 are combined with a hinge 2711 sothat an e-book reader 2700 can be opened and closed with the hinge 2711as an axis. With such a structure, the e-book reader 2700 can operatelike a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image whichconstitutes one screen or different images. In the case where thedisplay portion 2705 and the display portion 2707 display differentimages, for example, the display portion on the right side (the displayportion 2705 in FIG. 9C) can display text and the display portion on theleft side (the display portion 2707 in FIG. 9C) can display graphics.Any semiconductor device described in Embodiments 1 and 2 can be appliedto the display portion 2705, 2707. Further, the semiconductor devicedescribed in Embodiment 3 can be applied to a memory circuit in thehousing 2701, 2703. Since a change of the electrical characteristics ofany semiconductor device described in Embodiments 1 to 3 is suppressed,an e-book reader with high reliability can be provided.

Further, FIG. 9C illustrates an example in which the housing 2701 isprovided with an operation portion and the like. For example, thehousing 2701 is provided with a power switch 2721, operation keys 2723,a speaker 2725, and the like. With the operation key 2723, pages can beturned. A keyboard, a pointing device, or the like may also be providedin the same surface of the housing as the display portion. Further, anexternal connection terminal (e.g., an earphone terminal, or a USBterminal), a recording medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing.Further, the e-book reader may be equipped with a function of anelectronic dictionary.

The e-book reader may be configured to be able to transmit and receivedata wirelessly. Through wireless communication, desired book data orthe like can be purchased and downloaded from an electronic book server.

FIG. 9D illustrates a mobile phone which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 is provided with adisplay panel 2802, a speaker 2803, a microphone 2804, a pointing device2806, a camera lens 2807, an external connection terminal 2808, and thelike. The housing 2800 is provided with a solar cell 2810 for chargingthe mobile phone, an external memory slot 2811, and the like. Further,an antenna is incorporated in the housing 2801. Any semiconductor devicedescribed in Embodiments 1 and 2 can be applied to the display panel2802. Further, the semiconductor device described in Embodiment 3 can beapplied to a memory circuit in the housing 2800, 2801. Since a change ofthe electrical characteristics of any semiconductor device described inEmbodiments 1 to 3 is suppressed, a mobile phone with high reliabilitycan be provided.

The display panel 2802 is also provided with a touch panel in which aplurality of operation keys 2805 that is displayed as images isillustrated by dashed lines in FIG. 9D. Further, a boosting circuit bywhich a voltage output from the solar cell 2810 is increased to besufficiently high for each circuit is also provided.

In the display panel 2802, the display direction is appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus enables a videophone call. The speaker 2803 and themicrophone 2804 can be used for videophone calls, recording and playingsound, and the like as well as voice calls. Further, the housings 2800and 2801 in a state where they are developed as is in FIG. 9D can shiftby sliding so that one is lapped over the other, whereby the size of themobile phone can be reduced, which is suitable for carrying.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, which enables chargingand data communication with a personal computer or the like. Moreover, alarge amount of data can be stored by inserting a storage medium thereofinto the external memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be equipped.

FIG. 9E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Anysemiconductor device described in Embodiments 1 and 2 can be applied tothe display portion A 3057, the display portion B 3055. Further, thesemiconductor device described in Embodiment 3 can be applied to amemory circuit in the main body 3051. Since a change of the electricalcharacteristics of any semiconductor device described in Embodiments 1to 3 is suppressed, a digital video camera with high reliability can beprovided.

FIG. 9F illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. In this embodiment, the housing9601 is supported by a stand 9605. Any semiconductor device described inEmbodiments 1 and 2 can be applied to the display portion 9603. Further,the semiconductor device described in Embodiment 3 can be applied to amemory circuit in the housing 9601. Since a change of the electricalcharacteristics of any semiconductor device described in Embodiments 1to 3 is suppressed, a television set with high reliability can beprovided.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

The television set 9600 is provided with a receiver, a modem, and thelike. With the use of the receiver, general television broadcasting canbe received. Moreover, the display device can be connected to acommunication network with or without wires via the modem, wherebyone-way (from sender to receiver) or two-way (between sender andreceiver or between receivers) data communication can be performed.

Embodiment 4 can be implemented in appropriate combination with thestructures described in the other embodiments.

Example 1

In this example, MOS (metal oxide semiconductor) substrates weremanufactured, and results of withstanding voltage measurement and CV(capacitance vs. voltage) measurement performed thereon are describedusing FIGS. 10A, 10B, 11A, and 11B, FIG. 12, FIGS. 13A, 13B, 14A, and14B, and FIG. 15.

First, a method for manufacturing samples used in this example isdescribed.

In Sample A, a 50-nm-thick silicon oxide (SiOx) film was formed as afirst insulating film over a silicon (Si) substrate by a sputteringmethod. Next, a 10-nm-thick In—Ga—Zn—O—N-based metal oxide (IGZON) filmwas formed as a metal oxide film over the silicon oxide film by asputtering method. Then, a 50-nm-thick silicon oxide film was formed asa second insulating film over the In—Ga—Zn—O—N-based metal oxide film bya sputtering method.

In Sample B, the metal oxide film in Sample A was replaced from theIn—Ga—Zn—O—N-based metal oxide film to a 10-nm-thick In—Ga—Zn—O-basedmetal oxide (IGZO) film formed by a sputtering method. The otherstructure and the other manufacturing method of Sample B were the sameas those of Sample A.

In Sample C, a 50-nm-thick silicon oxynitride (SiON) film was formed asa first insulating film over a silicon substrate by a plasma enhancedCVD method. Next, a 10-nm-thick In—Ga—Zn—O-based metal oxide film wasformed over the silicon oxynitride film by a sputtering method. Then, a50-nm-thick silicon oxynitride film was formed as a second insulatingfilm over the In—Ga—Zn—O-based metal oxide film by a plasma enhanced CVDmethod.

In Sample D, the silicon oxide film (each of the first insulating filmand the second insulating film) in Sample A was replaced with a50-nm-thick silicon oxynitride film formed by a plasma enhanced CVDmethod. The other structure and the other manufacturing method of SampleD were the same as those of Sample A.

As Sample E, a 100-nm-thick silicon oxide film was formed over a siliconsubstrate by a sputtering method.

Next, heat treatment was performed on Samples A to E. The heat treatmentwas performed at 300° C. for one hour under a nitrogen atmosphere.

Next, in each of Samples A to E, a 400-nm-thick electrode (having anarea of 0.785 mm²) formed of an alloy of aluminum and titanium (AL-Ti)was formed over the second insulating film by a sputtering method.

Finally, heat treatment was performed on Samples A to E at 250° C. forone hour under a nitrogen atmosphere.

Structures of MOS substrates of Samples A to E thus obtained are shownin Table 1. Note that FIF, MOF, SIF and EL represent the firstinsulating film, the metal oxide film, the second insulating film andthe electrode, respectively.

TABLE 1 Sample Substrate FIF (nm) MOF (nm) SIF (nm) EL (nm) A Si SiOx(50) IGZON (10) SiOx (50) Al—Ti B SiOx (50) IGZO (10) SiOx (50) (400) CSiON (50) IGZO (10) SiON (50) D SiON (50) IGZON (10) SiON (50) E SiOx(100) — —

Next, current vs. voltage (I-V) characteristics of Samples A to E weremeasured. The measurement was performed at 13 points in each sample.

Results of the withstanding voltage measurement are shown in FIGS. 10A,10B, 11A, 11B, and 12. FIG. 10A shows results of Sample A; FIG. 10Bshows results of Sample B; FIG. 11A shows results of Sample C; FIG. 11Bshows results of Sample D; and FIG. 12 shows results of Sample E. InFIGS. 10A, 10B, 11A, 11B, and 12, the horizontal axis indicates voltageand the vertical axis indicates current.

It was found that Samples C and D shown in FIGS. 11A and 11Brespectively exhibit fast rising of current and have low withstandingvoltage. In contrast, it was found that Samples A and B shown in FIGS.10A and 10B respectively exhibit slower rising of current and havehigher withstanding voltage than Samples C and D. It was also found thatSample E shown in FIG. 12 has a withstanding voltage which is equivalentto those of samples A and B.

Next, CV (capacitance vs. voltage) measurement was performed on SamplesA to E. The measurement was performed at 4 points in each sample.

Results of the CV measurement are shown in FIGS. 13A, 13B, 14A, 14B, and15. FIG. 13A shows results of Sample A; FIG. 13B shows results of SampleB; FIG. 14A shows results of Sample C; FIG. 14B shows results of SampleD; and FIG. 15 shows results of Sample E. In FIGS. 13A, 13B, 14A, 14B,and 15, the horizontal axis indicates voltage and the vertical axisindicates capacitance value.

No CV curve could be obtained in any of Samples C and D shown in FIGS.14A and 14B respectively. This seems to be because the withstandingvoltage of the insulating film in Samples C and D is not high enough tokeep the capacitance as is seen from the results of FIGS. 11A and 11B.In contrast, Sample A shown in FIG. 13A, Sample B shown in FIG. 13B, andSample E shown in FIG. 15 exhibited good CV curves.

Further, it was found that the CV curves of Samples A and B shift in thepositive direction as compared with the CV curve of Sample E. This isbecause more negative fixed charges exist in Samples A and B than SampleE, suggesting that provision of such an insulating film so as to be incontact with a metal oxide film including a channel formation region ofa transistor enables the threshold voltage of the transistor to beshifted in the positive direction.

In Samples C and D, the silicon oxynitride films formed by a plasmaenhanced CVD method were used as insulating films by which the metaloxide film was sandwiched. Oxygen is not eliminated by heat treatmentfrom the silicon oxynitride film formed by a plasma enhanced CVD method.It seems that oxygen was not supplied from the insulating film to themetal oxide film, so that the metal oxide film was not able to beinsulated. On the other hand, in Samples A and B, the silicon oxidefilms formed by a sputtering method were used as insulating films bywhich the metal oxide film was sandwiched. It seems that oxygen waseliminated by heat treatment from the silicon oxide film formed by asputtering method and was supplied sufficiently to the metal oxide film,so that the metal oxide film could be insulated; this seems to bebecause the withstanding voltage of Samples A and B was improved.

The above results reveal that a metal oxide film being sandwiched byinsulating films from which oxygen is eliminated by heat treatmentfunctions as an insulating film.

Example 2

Described in this example are results of TDS analysis of the amount ofout-diffusion of oxygen passing through a metal oxide film from aninsulating film from which oxygen is released by heat treatment, in astructure where the metal oxide film is formed over the insulating film.

First, Samples F to I used in this example are described.

In Sample F, a 100-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method. Then, a 5-nm-thick In—Ga—Zn—O-basedmetal oxide film was formed over the silicon oxide film by a sputteringmethod.

In Sample G, a 100-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method. Then, a 10-nm-thick In—Ga—Zn—O-basedmetal oxide film was formed over the silicon oxide film by a sputteringmethod.

In Sample H, a 100-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method. Then, a 15-nm-thick In—Ga—Zn—O-basedmetal oxide film was formed over the silicon oxide film by a sputteringmethod.

In Sample I, a 100-nm-thick silicon oxide film was formed over a glasssubstrate by a sputtering method.

Next, TDS analysis was performed on Samples F to I. In this example, athermal desorption spectrometer EMD-WA1000S/W, manufactured by ESCO Ltd.was used to detect values of the amount of eliminated oxygen.

FIG. 16 shows results of the TDS analysis of Samples F to I.

As shown in FIG. 16, Sample I where only the silicon oxide film wasformed had a peak at about 200° C. On the other hand, almost no peak wasdetected in Samples F to H where the metal oxide film was formed overthe silicon oxide film.

The results shown in FIG. 16 revealed that out-diffusion of oxygencontained in a silicon oxide film can be prevented by forming a metaloxide film over the silicon oxide film. It was also indicated that themetal oxide film having a thickness of at least 5 nm preventsout-diffusion of oxygen contained in the silicon oxide film. Through theabove results, it was proved that a metal oxide film can prevent oxygenfrom passing therethrough.

Example 3

In this example, results of measurement of resistivity of metal oxidefilms are described using FIG. 17.

First, samples used in this example are described using FIG. 17.

(Condition 1)

The case where insulating films from which oxygen is not eliminated byheat treatment were used as insulating films by which a metal oxide film506 was sandwiched is referred to as Condition 1.

First, a 100-nm-thick silicon oxynitride film was formed as aninsulating film 502 over a glass substrate 500 by a plasma enhanced CVDmethod.

Next, a 100-nm-thick tungsten film was formed thereover by a sputteringmethod. Then, the tungsten film was subjected to a photolithographyprocess and an etching process to form electrodes 504 a and 504 b.

Next, an In—Ga—Zn—O-based metal oxide film was formed thereover as themetal oxide film 506 by a sputtering method. The conditions of formingthe metal oxide film were the following: a target whose compositionratio is In:Ga:Zn=1:1:1; Ar/O₂=30/15 sccm; a pressure of 0.4 Pa; a powerof 0.5 kW; a substrate temperature of 200° C.; and a film thickness of30 nm. After that, heat treatment was performed on the metal oxide film506 at 450° C. for one hour under a nitrogen atmosphere.

Next, a 100-nm-thick silicon oxynitride film was formed as an insulatingfilm 508 thereover by a plasma enhanced CVD method.

Next, the insulating film 508 and the metal oxide film 506 weresubjected to a photolithography process and an etching process to formopenings reaching the electrode 504 a, 504 b.

Finally, heat treatment was performed on the sample at 350° C. for onehour under a nitrogen atmosphere.

(Condition 2)

The case where insulating films from which oxygen is eliminated by heattreatment were used as insulating films by which a metal oxide film 506was sandwiched is referred to as Condition 2.

First, a 100-nm-thick silicon oxide film was formed as an insulatingfilm 502 over a glass substrate 500 by a sputtering method.

Next, a 100-nm-thick tungsten film was formed thereover by a sputteringmethod. Then, the tungsten film was subjected to a photolithographyprocess and an etching process to form electrodes 504 a and 504 b.

Next, an In—Ga—Zn—O-based metal oxide film was formed thereover as themetal oxide film 506 by a sputtering method. The conditions of formingthe metal oxide film were the following: a target whose compositionratio is In:Ga:Zn=1:1:1; Ar/O₂=30/15 sccm; a pressure of 0.4 Pa; a powerof 0.5 kW; a substrate temperature of 200° C.; and a film thickness of30 nm. After that, heat treatment was performed on the metal oxide film506 at 450° C. for one hour under a nitrogen atmosphere.

Next, a 100-nm-thick silicon oxide film was formed as an insulating film508 thereover by a sputtering method.

Next, the insulating film 508 and the metal oxide film 506 weresubjected to a photolithography process and an etching process to formopenings reaching the electrode 504 a, 504 b.

Finally, heat treatment was performed on the sample at 350° C. for onehour under a nitrogen atmosphere.

(Condition 3)

The case where an insulating film from which oxygen is eliminated byheat treatment was used as the insulating film 502, and an insulatingfilm from which oxygen is not eliminated by heat treatment was used asthe insulating film 508 by which a metal oxide film 506 was sandwichedis referred to as Condition 3.

First, a 100-nm-thick silicon oxide film was formed as the insulatingfilm 502 over a glass substrate 500 by a sputtering method.

Next, a 100-nm-thick tungsten film was formed thereover by a sputteringmethod. Then, the tungsten film was subjected to a photolithographyprocess and an etching process to form electrodes 504 a and 504 b.

Next, an In—Ga—Zn—O-based metal oxide film was formed thereover as themetal oxide film 506 by a sputtering method. The conditions of formingthe metal oxide film were the following: a target whose compositionratio is In:Ga:Zn=1:1:1; Ar/O₂=30/15 sccm; a pressure of 0.4 Pa; a powerof 0.5 kW; a substrate temperature of 200° C.; and a film thickness of30 nm. After that, heat treatment was performed on the metal oxide film506 at 450° C. for one hour under a nitrogen atmosphere.

Next, a 100-nm-thick silicon oxynitride film was formed as theinsulating film 508 thereover by a plasma enhanced CVD method.

Next, the insulating film 508 and the metal oxide film 506 weresubjected to a photolithography process and an etching process to formopenings reaching the electrode 504 a, 504 b.

Finally, heat treatment was performed on the sample at 350° C. for onehour under a nitrogen atmosphere.

(Condition 4)

The case where an insulating film from which oxygen is not eliminated byheat treatment was used as the insulating film 502, and an insulatingfilm from which oxygen is eliminated by heat treatment was used as aninsulating film 508 by which a metal oxide film 506 was sandwiched isreferred to as Condition 4.

First, a 100-nm-thick silicon oxynitride film was formed as theinsulating film 502 over a glass substrate 500 by a plasma enhanced CVDmethod.

Next, a 100-nm-thick tungsten film was formed thereover by a sputteringmethod. Then, the tungsten film was subjected to a photolithographyprocess and an etching process to form electrodes 504 a and 504 b.

Next, an In—Ga—Zn—O-based metal oxide film was formed thereover as themetal oxide film 506 by a sputtering method. The conditions of formingthe metal oxide film were the following: a target whose compositionratio is In:Ga:Zn=1:1:1; Ar/O₂=30/15 sccm; a pressure of 0.4 Pa; a powerof 0.5 kW; a substrate temperature of 200° C.; and a film thickness of30 nm. After that, heat treatment was performed on the metal oxide film506 at 450° C. for one hour under a nitrogen atmosphere.

Next, a 100-nm-thick silicon oxide film was formed as the insulatingfilm 508 thereover by a sputtering method.

Next, the insulating film 508 and the metal oxide film 506 weresubjected to a photolithography process and an etching process to formopenings reaching the electrode 504 a, 504 b.

Finally, heat treatment was performed on the sample at 350° C. for onehour under a nitrogen atmosphere.

Next, the conductivity a was measured at 4 points of each of respectivesamples in Conditions 1 to 4. The 4-point average resistivity pcalculated from the measured conductivity a is listed in Table 2.

TABLE 2 Resistivity ρ [Ω · cm] Condition 1  1.4 × 10⁻² Condition 2 7.4 ×10⁹ Condition 3 8.6 × 10³ Condition 4 8.5 × 10⁶

As shown in Table 2, the resistivity p of the metal oxide film inCondition 1 was 1.4×10⁻² Ω·cm; the resistivity p of the metal oxide filmin Condition 2 was 7.4×10⁹ Ω·cm; the resistivity p of the metal oxidefilm in Condition 3 was 8.6×10³ Ω·cm; and the resistivity p of the metaloxide film in Condition 4 was 8.5×10⁶ Ω·cm.

According to the results of Condition 1, the resistance of the metaloxide film 506 once reduced by heat treatment after formation of themetal oxide film 506 is not changed to be kept to be low even byperforming heat treatment after formation of the insulating film 508,whereby the metal oxide film 506 is turned into a conductor.

According to the results of Condition 2, the resistance of the metaloxide film 506 once reduced by heat treatment after formation of themetal oxide film 506 is increased by performing heat treatment afterformation of the insulating film 508. This seems to be because oxygenvacancies generated in the metal oxide film are repaired by oxygensupplied from the insulating films 502 and 508. Consequently, the metaloxide film 506 is turned into an insulator (shows insulatingcharacteristics).

According to the results of Conditions 3 and 4, the resistance of themetal oxide film 506 is changed to be higher than that in Condition 1and lower than that in Condition 2 by heat treatment after formation ofthe insulating film 508, whereby the metal oxide film 506 remains asemiconductor.

Through the above results, it was revealed that the resistance of ametal oxide film can be adjusted by the kind of an insulating film whichis in contact with the metal oxide film (or the amount of oxygeneliminated from the insulating film).

This application is based on Japanese Patent Application serial no.2011-078111 filed with Japan Patent Office on Mar. 31, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a gateelectrode; a gate insulating film; a first metal oxide film; apassivation film over the first metal oxide film, wherein thepassivation film comprises a first insulating film, a second metal oxidefilm, and a second insulating film, wherein the first metal oxide filmcomprises all metal elements contained in the second metal oxide film,wherein the gate insulating film is sandwiched between the gateelectrode and the first metal oxide film, wherein the first metal oxidefilm is a semiconductor, wherein the second metal oxide film issandwiched between the first insulating film and the second insulatingfilm, and wherein the first insulating film is in contact with the firstmetal oxide film.
 2. The semiconductor device according to claim 1,further comprising a source electrode and a drain electrode which are incontact with the first metal oxide film; wherein the first metal oxidefilm is over the source electrode and the drain electrode, and whereinthe passivation film is over the source electrode and the drainelectrode.
 3. The semiconductor device according to claim 1, wherein athickness of the first insulating film is greater than a thickness ofthe second insulating film.
 4. The semiconductor device according toclaim 1, wherein a thickness of the first metal oxide film is greaterthan a thickness of the second metal oxide film.
 5. The semiconductordevice according to claim 1, wherein a thickness of the second metaloxide film is greater than or equal to 5 nm and less than or equal to 15nm.
 6. The semiconductor device according to claim 1, wherein all metalelements contained in the first metal oxide film are the same as the allmetal elements contained in the second metal oxide film.
 7. Asemiconductor device comprising: a gate electrode; a gate insulatingfilm; a first metal oxide film; a passivation film over the first metaloxide film, wherein the passivation film comprises a first insulatingfilm, a second metal oxide film, and a second insulating film, whereinthe first metal oxide film comprises all metal elements contained in thesecond metal oxide film, wherein the gate insulating film is sandwichedbetween the gate electrode and the first metal oxide film, wherein eachof the first metal oxide film and the second metal oxide film containsat least two kinds of elements selected from In, Ga, Sn, and Zn, whereinthe first metal oxide film is a semiconductor, wherein the second metaloxide film is sandwiched between the first insulating film and thesecond insulating film, and wherein the first insulating film is incontact with the first metal oxide film.
 8. The semiconductor deviceaccording to claim 7, further comprising a source electrode and a drainelectrode which are in contact with the first metal oxide film; whereinthe first metal oxide film is over the source electrode and the drainelectrode, and wherein the passivation film is over the source electrodeand the drain electrode.
 9. The semiconductor device according to claim7, wherein a thickness of the first insulating film is greater than athickness of the second insulating film.
 10. The semiconductor deviceaccording to claim 7, wherein a thickness of the first metal oxide filmis greater than a thickness of the second metal oxide film.
 11. Thesemiconductor device according to claim 7, wherein a thickness of thesecond metal oxide film is greater than or equal to 5 nm and less thanor equal to 15 nm.
 12. The semiconductor device according to claim 7,wherein all metal elements contained in the first metal oxide film arethe same as the all metal elements contained in the second metal oxidefilm.
 13. A semiconductor device comprising: a gate electrode; a gateinsulating film; a first metal oxide film; a first insulating film overthe first metal oxide film, and a second metal oxide film over the firstinsulating film, wherein the first metal oxide film comprises all metalelements contained in the second metal oxide film, wherein the gateinsulating film is sandwiched between the gate electrode and the firstmetal oxide film, wherein the first metal oxide film is a semiconductor,and wherein the first insulating film is in contact with the first metaloxide film.
 14. The semiconductor device according to claim 13, furthercomprising a source electrode and a drain electrode which are in contactwith the first metal oxide film; wherein the first metal oxide film isover the source electrode and the drain electrode, and wherein the firstinsulating film is over the source electrode and the drain electrode.15. The semiconductor device according to claim 13, wherein a thicknessof the first metal oxide film is greater than a thickness of the secondmetal oxide film.
 16. The semiconductor device according to claim 13,wherein a thickness of the second metal oxide film is greater than orequal to 5 nm and less than or equal to 15 nm.
 17. The semiconductordevice according to claim 13, wherein all metal elements contained inthe first metal oxide film are the same as the all metal elementscontained in the second metal oxide film.
 18. The semiconductor deviceaccording to claim 1, wherein the second metal oxide film is aninsulator.
 19. The semiconductor device according to claim 7, whereinthe second metal oxide film is an insulator.
 20. The semiconductordevice according to claim 13, wherein the second metal oxide film is aninsulator.
 21. A semiconductor device comprising: a gate electrode; agate insulating film; a first metal oxide film; a first insulating filmover the first metal oxide film, and a second metal oxide film over thefirst insulating film, wherein the first metal oxide film comprises Inand Zn, and the second metal oxide film comprises In and Zn, wherein thegate insulating film is sandwiched between the gate electrode and thefirst metal oxide film, wherein the first metal oxide film is asemiconductor, and wherein the first insulating film is in contact withthe first metal oxide film.
 22. The semiconductor device according toclaim 21, further comprising a source electrode and a drain electrodewhich are in contact with the first metal oxide film; wherein the firstmetal oxide film is over the source electrode and the drain electrode,and wherein the first insulating film is over the source electrode andthe drain electrode.
 23. The semiconductor device according to claim 21,wherein all metal elements contained in the first metal oxide film arethe same as all metal elements contained in the second metal oxide film.24. The semiconductor device according to claim 21, wherein the secondmetal oxide film is an insulator.